Wireless control networks have recently become a ubiquitous trend in the field of communication, especially for building management systems. Wireless technologies present major advantages in terms of freedom of device placement, device portability, and installation cost reduction, since there is no need for drawing cables and drilling. Thus, such technologies are particularly attractive for interconnecting, detecting, automation, control or monitoring systems using sensor devices such as light switches, light dimmers, wireless remote controllers, movement or light detectors, that have to be set up in distant places one from the other and from the devices they control, e.g. lights.
One of the drawbacks appearing in networks of the like relates to device powering. Indeed, since the devices are not wired, they can not anymore receive power necessary for performing all the operations required in the network from the mains or via the connection with the controller. Thus, it has been envisaged to equip such devices with built-in batteries. However, since the devices are quite size-constrained, batteries may not be of a large size, which results either in a reduced device lifetime, or in labour intensive battery replacement.
It has been suggested to remedy this issue by equipping sensor devices with self-sustained energy sources that harvest energy from its environment. Still, the amount of energy achievable by off-the-shelf energy harvesters is very limited, which means that the features and functions of the batteryless devices are heavily restricted.
Among the functions that are mandatory to be maintained for good operation in a wireless network is the link connection, which makes it possible to ensure at any time that a resource restricted device is linked to a router which forwards messages on its behalf. In existing implementations therefore, a parent-child relationship is established between an end device, generally resource-restricted, and its parent router. The child end device addresses all its communication to the parent for being forwarded to their final destination. However, in case of energy-harvesting device, this relationship creates a single point of failure in the network, because if the parent link is broken, communication from the end device can not be successfully performed anymore. Moreover, in most cases, such a parent link failure may not even be detected by the end device, due to non-existent or not used receiving circuit on the resource-restricted device, or insufficient energy to wait for and receive the feedback. Indeed, since the end device has very limited resources, it can not perform a complete search in order to find a new parent router when the communication is lost, thus operation in the network is compromised, as well as the operation of the end device from the user's perspective.
So as to remedy the issue of single point of failure created by the parent link of resource-restricted devices, solutions have been proposed using MAC-level broadcast communication. In such methods, a source device only transmits data, without tracking whether it gets forwarded by its neighbours, and without expecting any acknowledgment. On the higher layers, forwarding is taken care of, while also the potential duplicates may be filtered out or prevented by additional mechanisms. The reliability is thus achieved by allowing several nodes, with different propagation conditions, to receive and, if required, forward the frame. One of such higher layer mechanisms is 802.15.4/ZigBee network-level multicast communication: each node within a given range receives the message, and each group member forwards it, once or several times. Another such higher layer mechanism is 802.15.4/ZigBee network-layer broadcast. However, its reliability is based on passive acknowledgement mechanism which requires the originator, as well as any other forwarding node, to track whether its neighbours forward the message and re-try it, if some didn't.
Both mechanisms imply high energy consumption, e.g resulting from response tracking and/or the possible retries that can not be supported by end devices. Furthermore, the usage of broadcast/multicast results in high bandwidth consumption, due to multiple devices in a given neighbourhood re-transmitting several times, which may again lead to network overload and as a consequence, in reduced reliability or temporary failure from a user point of view.